If you have those then you should know that, taking x to be the distance from the equilibrium position, positive above, negative below, x= -0.1 cos(2pi t/.4). To answer the question "how long after being released is the posiiton of the mass equal to 0.05m", set x= 0.05 and solve for t.

By the way, do you understand that saying "period T= .4 frequency f= 2.5" doesn't mean anything unless you give the units?